Irradiation of nitro-PAHs in solution at ambient conditions leads to formation of its lowest excited triplet, dissociation intermediates nitrogen oxide (NO center dot) and aryloxy radical (Ar-O-center dot). Experimental and theoretical studies demonstrated that Franck-Condon excited singlet state S-FC(pi pi*) to a receiver, higher-energy triplet state T-n(n pi*) controlled the ultrafast population of the triplet state and, hence, the slight fluorescence yield of nitronaphthalenes. However, the detailed information about the curve crossings of potential energy surfaces and the major channels for forming T-1 species and Ar-O-center dot radical were unclear. Here, by using the CASSCF//CASPT2 method, an efficient decay channel is revealed: S2-FC-1NN -> S2-MIN-1NN -> or S2T3-MIN-1NN -> T3-MIN-1NN or T3T2-MIN-1NN -> T2-MIN-1NN or T2T1-MIN-1NN -> T1-MIN-1NN. This explains the high yield of T1-1NN species and minor yield of Ar-O-center dot and NO center dot radicals. The calculation results suggest the bifurcation processes take place predominantly after the internal conversion to the T1-1NN state via T2T1-MIN-1NN one leads to T1-MIN-1NN, while the other to T1-MIN-ISO to produce Ar-O-center dot and NO center dot radicals.